A sparse index in databases is a file with pairs of keys and pointers for every block in the data file, while a dense index in databases is a file with pairs of keys and pointers for every record in the data file.
A clustered index, is a index on a non-key field of a table. The records are physically ordered on this no-key field.
Since it is a non-key field, so there may be multiple records against a single key. Hence, a key and a pointer to a first record of the block (which contain matching records against that key) is stored.
Thus it is a Spare Index.
I could find one example on internet explaining the same
As can be seen above, clustered index is not containing pointer to every record in file. It is containing record to a block in a file.
On the other hand, dense index, which is unclustered, is holding record to every record in a file.
Also, note, CREATE TABLE... creates clustered index, while CREATE INDEX creates
unclustered index.
Reference:
http://jcsites.juniata.edu/faculty/rhodes/dbms/sortdfiles.htm